The present invention relates to composite cans and, more particularly, to composite cans having an outer covering or label which is removable and/or provides burst resistance to the can.
Composite containers are widely used commercially for packaging various products such as snack foods, refrigerated dough products, and the like. Typically, the containers are formed of a helically or convolutely wrapped paperboard or boardstock layer, an inner liner layer providing a moisture barrier function, and an exterior layer that usually forms a label for printed indicia and/or graphics. The various layers of the container are wrapped in strip form onto a mandrel and are secured together by an adhesive applied between the confronting faces of the adjacent layers.
Water-based or xe2x80x9cwetxe2x80x9d adhesives have become popular because of concerns that the use and disposal of solvent-based adhesives may cause harm to the environment. Conventionally, a wet adhesive is applied to substantially the entire surface of a strip. The water in the adhesive has a tendency to permeate the paperboard body wall and, as a result, the stiffness and dimensional stability of the tubular body wall are compromised.
Consequently, it has been necessary in many cases to hold completed cans in storage for a period of time prior to being filled with product and having end closures seamed on the cans, so that the cans have time to dry to increase their strength sufficiently to be able to tolerate the stresses imposed on them during the filling and seaming operations. Without this holding period, the cans are more prone to being damaged during these subsequent manufacturing steps.
As the line speeds of filling plants continue to increase through efforts to improve efficiency and productivity, the line speeds of container-making plants must also increase to meet the increased demand for containers. Accordingly, because of the reduced time available for the xe2x80x9cgreenxe2x80x9d (i.e., freshly made and still wet) tubes to set up and gain strength, it has become increasingly more difficult for parent tubes to go through secondary operations such as cutting the tubes to can lengths, flanging the cans, seaming end closures on the cans, etc., without being damaged.
In the case of composite containers for refrigerated dough products, the above-noted problems are exacerbated by the use of low-tack adhesive for affixing the exterior label layer to the paperboard body wall of the can. The low-tack adhesive is typically used in order to permit the label to be removed substantially in one piece so that baking instructions or the like that are printed on the label are not destroyed during opening of the can. In such dough cans, typically the paperboard body wall is spirally wound such that its edges form a helical butt joint that is easily opened to gain access to the dough product. The butt joint is held closed by the exterior label adhered to the paperboard body wall. Thus, the strength and integrity of the can is dependent largely on the strength of the label and the adherence of the label to the body wall. When a low-tack adhesive is used for affixing the label, the green strength of the can is still further reduced, relative to a can made with high-tack adhesive. Additionally, the burst strength of the can is compromised.
The present invention enables increased green strength and dimensional stability of composite cans so that parent tubes can undergo secondary operations with less susceptibility to being damaged, and so that the holding period for green cans may be reduced or eliminated. The invention, in preferred embodiments, also enables enhanced performance of composite cans, such as improved burst strength of dough cans while still permitting intact label removal.
To these ends, the invention provides composite cans and methods of making such cans, in which a body wall is constructed from one or more strips of paperboard wrapped about an axis of the can to form a tubular can body, and a exterior cover layer is applied to cover the body wall, and wherein adhesive is applied between the adjoined surfaces of the cover layer and the body wall in a predetermined pattern providing substantially less than 100 percent coverage of the surfaces. The invention thus facilitates a reduction in the total amount of adhesive applied to the body wall relative to conventional techniques employing full-coverage coating of adhesive onto paperboard strips. Consequently, there is less moisture available to permeate the paperboard body wall, so that green strength is improved.
In some preferred embodiments of the invention, the adhesive pattern includes areas of relatively denser coverage and areas of relatively less dense coverage on an adhesive weight per unit area basis. Such nonuniform patterns enable the performance of the composite can to be enhanced in different ways depending on the selected pattern. For example, in one preferred embodiment of the invention, an easy-open composite can of the type used for refrigerated dough comprises a spirally wound liner strip and a body wall formed of a single paperboard strip spirally wound onto the liner in edge-abutting relation so as to form a butt joint that spirally extends lengthwise along the can body. The can includes a cover layer comprising a web-like strip spirally wound onto the body wall with edges of the web-like strip offset from the butt joint. In order to reinforce the butt joint and improve burst strength of the can while still permitting the cover to be removed substantially intact, the adhesive is applied in a relatively higher-density pattern between the body wall and the cover layer in areas of the cover layer adjacent the butt joint, and the adhesive is applied in a relatively lower-density pattern in areas of the cover layer away from the butt joint.
Preferably but not necessarily, adhesive is applied with a relatively high-density pattern in a continuous stripe along the cover layer so that the continuous stripe is adjacent the butt joint of the body wall. In another preferred embodiment, two continuous stripes of adhesive are applied spaced apart slightly so that the two stripes are adjacent to and spaced from the butt joint on opposite sides thereof.
Other aspects of can performance can also be enhanced. For instance, in accordance with one preferred embodiment of the invention, dog-earing (i.e., lifting of an edge of the liner and/or the label) on cut ends of tubes can be reduced by applying a continuous stripe of adhesive along an edge of the liner strip and/or the label strip.
In conventional composite dough can-making processes, dextrin adhesives are commonly used for affixing the label to the can because they have relatively low tack, thus permitting intact label removal, while also providing adequate burst strength under most environmental conditions. It has been found, however, that in very high-humidity environments (e.g., relative humidity greater than 90 percent), the dextrin adhesive can begin to be dissolved by the moisture such that that the label can slip and the container can burst. Furthermore, at somewhat lower relative humidities up to about 90 percent, the peel strength of the label is a strong function of the relative humidity, and at the upper end of this range the peel strength can be so high that it is difficult or impossible to remove the label intact.
To address these problems, in preferred embodiments of the invention, the adhesive comprises a water-insoluble adhesive, examples of which include but are not limited to polyvinyl acetate adhesive (PVA), hot melt adhesives, and acrylates. More preferably, the adhesive is a PVA adhesive. PVA has relatively high tack and, accordingly, has not heretofore been used in commercial can-making processes for affixing labels to easy-open cans because of the desire that the label be removable intact. With conventional flood-coating of adhesive onto the plies, a label affixed with PVA adhesive would be difficult or impossible to remove intact. However, in accordance with the present invention, PVA is applied with a low coat weight relative to a conventional adhesive such as dextrin adhesive, and thus intact label removal is facilitated. Preferably, PVA adhesive is applied in a pattern covering substantially less than 100 percent of the paperboard body wall. The bond strength of PVA adhesive is not substantially influenced by the presence of moisture, and therefore the bond strength of the label is more stable over a range of environment conditions including high-humidity environments.
Various types of adhesive patterns can be used, and the adhesive can be applied in various ways. In one preferred embodiment of the invention, a textured adhesive applicator roll is used for applying adhesive to the label strip. The roll may be etched or machined with the desired pattern, for example, an intersecting grid pattern, a stripe pattern, etc.
The invention thus enables moisture add-on to be reduced and can performance to be enhanced through application of adhesive to plies in a partial-coverage pattern and, in some embodiments, through selective application of adhesive with different adhesive densities to different areas of a ply in accordance with the desired bond strength in those areas. With respect to easy-open composite cans, the invention enables high-tack water-insoluble adhesives to be used for application of the label such that good burst-resistance is achieved and intact label removal can be accomplished at the same time.